In some arc welding systems, the welding machines require some means of non-contact arc initiation and re-ignition. For example, welding power supplies may include a high frequency ignition circuit (such as a capacitor discharge circuit or a spark gap tank circuit) to assist in bridging the gap from the electrode to the workpiece. When welding in AC mode, arc stability is an issue because there is a possibility the arc may not re-light each time the waveform goes through zero current when the polarity is switched. Indeed, it is nearly impossible to prevent the arc from going out during a zero transition. The typical arc voltage is usually less than 30 volts and a much higher voltage is required to reliably sustain the arc under all conditions. Depending on the process, the required voltage may be up to 100 volts or more. In a typical system, elevated voltages are transiently supplied by various passive elements in the welding circuit such as the output choke, but are generally limited to the Open Circuit Voltage (OCV) of the power supply. As discussed above, AC welding is a special case because the arc extinguishes at each polarity reversal, and the OCV of the power source is typically not enough to reestablish the arc. Generally, the OCV of the power supply must be as high as 100 volts OCV in order to sustain the arc. However, at reverse polarity, there is only about 50 volts OCV plus any voltage from the output choke to reestablish the arc. This voltage is typically not enough to reestablish the arc. Accordingly, the challenge is to provide a means to re-ignite the arc after the welding waveform passes through the zero transition. To this end, a number of methods exist that impose an elevated voltage across the welding load to re-ignite the hot ionized gas as the waveform transitions to the opposite polarity.
For example, high frequency ignition circuits can be turned on each time the AC waveform changes polarity. The high frequency ignition circuit induces high voltage, high frequency pulses that generate sparks between the electrode and workpiece to initiate the arc. The high frequency ignition circuit is a reliable method of re-igniting the arc. However, the electrical noise from these high frequency ignition circuits can damage the electronics in modern welding power supplies and other sensitive equipment located in the vicinity or workplace. Other methods include a superposition or a center tapped choke to reestablish the arc when the polarity is switched. Because these methods do not use high voltage, high frequency pulses, the sensitive electronics are less susceptible to getting damaged by the electrical noise. However, in extreme conditions, the welding process may still experience a “pop out,” i.e., the arc does not re-ignite. Further, once the energy in the center-tapped choke circuit is released, the process will need to wait until it charges again. Additional information concerning superposition circuits and center-tap choke circuits can be found in U.S. Pat. No. 7,385,159, which is incorporated by reference herein in its entirety as background material.
In addition to the above, prior art methods do not provide for low current stability and waveform smoothing. The physical nature of the arc is influenced by properties such as the material being welded, weld puddle size, heat input, etc. and the voltage or length of the arc can be significantly high. In such cases, the voltage necessary to sustain the arc can instantaneously exceed the voltage provided by the power supply and the arc will extinguish.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such approaches with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.